Herrick, i\IorphoIogy of Nervous System. 171 



thalamus and passes toward the tract of the third nerve, 

 passing Meynert's bundle, and apparently unites with fibres 

 passing toward the cerebrum. If these fibres be compared 

 with the fornix tract, they might seem to warrant the 

 suggested homology of the hypoaria with the mammillaria. 

 This seems the more reasonable inasmuch as there seems 

 to be a homologue of the corpus geniculatum dorsad 

 to them. 



The third ventricle varies somewhat from the form found 

 in Acipenser. In the region of the infundibulum it is a long 

 slit, opening above into the dorsal sac and connecting by 

 a narrow triangular opening dorsad with the cephalic section 

 of the ventricle. Ventrad, there is a short cephalad 

 diverticle, so that transections in front of the habenae present 

 the appearance of having the two halves connected by a soft 

 commissure. After passing for a short distance cephalad, the 

 ventricle again passes to the ventral surface and extends for- 

 ward into the supra-chiasmic fossa. It would appear that 

 all that would be necessary to produce a real soft commissure 

 would be the opening of a communication between the ven- 

 tral portion of the two segments of the third ventricle. 

 Caudad, the ventricle gives rise to the ventro-median ventricle 

 of the saccus vasculosus and two lateral ventricles of the 

 hypoaria. 



The third and fourth nerves have the usual relations, 

 though somewhat complicated by the inward growth of the 

 cerebellum. In the sturgeon (Plate XI, Fig. 7) the third 

 nerve can be traced, from its nidulus near the floor of the 

 aqueduct not far from the median line, ventrad to its exit, 

 from which point it passes laterad. In Lepidosteus it pursues 

 a more curved course and emerges in the space between the 

 hypoaria and dorsad to the overlapping part of the cinereum, 

 thence arching caudo-laterad about the neck of the hypoaria, 

 and turning cephalad, it occupies a small groove in the lateral 

 aspect of the thalamus dorsad to the hypoarium. 



The fourth nerve crogges in the valve and escapes in the 



